Book covers, folders and booklets of the kind described above are already known. They generally comprise a cardboard or plastics material, which is provided with a spine and two covers joined thereto via two crease lines, said covers enclosing a plurality of sheets of paper. In producing the blank comprising covers and spine, a sheet or roll of material is cut to the desired format, the blank being provided with two crease lines defining the spine. In order that said sheets of paper shall be attached to the spine, it is coated with a bonding agent or binder, subsequent to which the sheets of paper are pushed into the binder, which is then allowed to harden.
Binders of thermosetting type have recently begun to be used to rationalize the manufacture of folders and booklets. Such a binder is in a solid state at room temperature, and is supplied in large sheets or rolls from which strips are cut. A strip is attached to the inside of the spine by placing it between the crease lines and thereafter heating it so that the binder melts and adheres to the inside of the spine. When the binder has hardened, the book covers are taken to a binding machine together with the pages which are to be enclosed between the covers, with the edges of the sheets in contact with the strip attached to the inside of the spine. The strip is heated by the machine, the edges of the sheets being surrounded by viscous binder. After cooling the sheets are rigidly attached to the spine.
The above-described method of manufacturing folders or booklets is comparatively effective, but necessitates a plurality of work operations. Furthermore, during the process it is difficult to orient the strip exactly on the inside of the spine between the crease lines, which is necessary if all the sheets along the whole of their length are to make contact with the strip and adhere firmly thereto, and if the crease lines are to be kept free from binder and the covers are to be bent as intended, without any obstruction.